The invention relates to a circuit arrangement for amplifying a periodic signal and for adjusting a reference level contained therein, level a limit value stage in which the amplified signal is compared to an at least substantially constant limit value and which alternately switches a charging/discharging arrangement into either of two positions, depending on whether the value of the periodic signal exceeds or falls short of the limit value, as the case may be. The charging/discharging arrangement supplies currents for alternately charging or discharging a storage means such that a mean value at least substantially corresponding to the reference level occurs at the storage means. The circuit further comprises an amplifier stage including at least an amplifier component for amplifying the signal to which is applied the mean value of the storage means for adjusting the reference level contained in the signal.
Such a circuit arrangement is disclosed in the German patent Ser. No. DE 3.214.756 and its equivalent U.S. Pat. No. 4,470,066 for example FIG. 3 or 4 with the associated descriptions. Such circuit arrangements are preferably used as clamping circuits for processing television signals in television receivers. In contradistinction to, what is commonly referred to as, forced clamping, in which case the television signal is sampled by an external clamping pulse at short time intervals, the circuit arrangements of the type mentioned in the opening paragraph do not require an external clamping pulse and are essentially less sensitive to disturbances in the television signal. The reference level in the television signal to be adjusted can be chosen in a simple manner by fixing the ratio of the charging current to the discharging current of the storage means. Besides, for a trouble-free functioning of the circuit arrangement it is essential that this ratio between the charging and discharging currents, once it has been chosen, should not be subjected to fluctuations, as these fluctuations may be manifested as fluctuations in the reference level to be adjusted, that is to say, in a mean value occurring at the storage means.
Such fluctuations can, for example, be caused by the input current of an amplifier stage comprised in the circuit arrangement and connected to the storage means, in the case when this input current fluctuates with a signal being periodic but having a changing amplitude. Furthermore, fluctuating currents may occur as a result of production tolerances in the amplifier stage or as a result of temperature variations, specially affecting the gain factors of the amplifier components comprised in the amplifier stage. It has been shown that the latter effect can have significant repercussions.
To reduce the influence of the said fluctuations, the current for discharging the storage means therefore should be chosen to be considerably larger than the current originating from the amplifier stage and subject to said fluctuations. If, for example, the input current of the amplifier stage cannot be chosen to be as small as desired, in order not to narrow down the bandwidth of the periodic signal amplified by the amplifier stage more than allowed, a minimum value for the current for discharging the storage means is given too. This also requires a minimum value of the charging current and of the storage means, for example a capacitor, so as to be able to effect the charging of the storage means and to keep a variation in the charge on the storage means as a result of the discharging current, during a signal period, at the lowest possible value as such discharging processes distort the variation of the periodic signal.
If, for example, the periodic signal is a television signal, in which picture synchronizing signals occur in addition to line synchronizing signals determining the period, too large a deviation from the determined reference level would occur if the storage means, for example a capacitor, were too small.
On the other hand, large currents require a larger dimensioning of all components through which they pass. Storage means designed as capacitors of specific minimum dimensions particularly require a minimum floor space. Consequently, integrating the described circuit arrangement on a wafer will be harder.